As known, coffee espresso machines comprise a boiler to hold and heat the water, a device for water pressurization, one or more devices to supply the coffee and other internal and external components. All these components are contained, preferably, inside an external casing.
The boiler is usually connected to a cold water intake pipe, used to reset, preferably in an automatic way, the optimum level of water during operation. On the water intake there is an electrically activated valve controlled by a dedicated monitoring device which provides the monitoring of the water level inside the boiler and the activation of the electrically activated valve at the detection of the minimum level until the desired level is restored.
The Applicant's publication number WO0160221, teaches a capacitive device to regulate the level of a liquid inside a boiler of a coffee espresso machine.
The known device particularly comprises a capacitive sensor in which a small metal spring acts as the first plate of a capacitor, a tube connecting the boiler acts as the dielectric of the capacitor, and the water (being electrically conductive) acts as the second plate of the capacitor.
According to the known art, the second plate is grounded through the metal of the boiler. A control circuit is connected to the first plate and to the mass of the boiler and it is designed to measure the capacitive variations between the small spring, which can be positioned in pre-established positions along the transparent tube, and the mass of the boiler, at the variation of the level of water inside the transparent tube. As known, the presence of air in correspondence of the small spring is detected as a lower capacitance with respect to the condition when the water is present in correspondence of the small spring.
A problem of the known art is that the equivalent circuit, made of the small spring (capacity sensor), the transparent tube, the air and/or water, the structure and mass of the boiler, present a total impedance, of which the variations caused by the presence of air or water in correspondence of the small spring can be irrelevant compared to the impedance caused by the column of water inside the transparent tube, to the position of the small spring along the transparent tube, and to the structure of the boiler.
In fact, the impedance variations of the water, in relation to its characteristics, such as temperature, mineral salt content, etc., and the characteristics of the boiler, after oxidation, etc., are such as to become relevant in respect to the capacitive variations whose values need to be measured to determine precisely the level of water inside the transparent pipe.
Furthermore, the Applicant has noticed that the performance of the known device is greatly affected by the uncertainty of the capacitive impedance caused by the characteristics of the water, the boiler and the position of the capacitive sensor. Particularly, the Applicant noticed that, above all, the variations of the capacitive impedance of the water, and, secondarily, those of the boiler, can be compared to the impedance variations to be measured to determine the water level in the transparent pipe and, therefore, inside the boiler.